1. Field of the Invention
The present invention relates to an X-ray computer tomography apparatus which dynamically changes an imaging condition during a scan on the basis of the scanogram data obtained by scanography executed before the start of the scan.
2. Description of the Related Art
X-ray computer tomography apparatuses (to be referred to as “X-ray CT apparatuses” hereinafter) have made remarkable progress after they were invented. Recently, X-ray CT apparatuses have been put into practice, which allow imaging schemes such as helical scan in which an object to be examined is helically irradiated with X-rays by helically and continuously rotating an X-ray tube while moving an object to be examined in the body axis direction and multi-slice scan which can execute tomography with respect to a plurality of slices by using a plurality of detection arrays. It may safely be said that such progress in X-ray CT apparatuses is the result of relentless challenges to an improvement in image quality, reductions in imaging time and image reconstruction time, a reduction in X-ray dose, and the like.
A general X-ray CT apparatus called a third-generation apparatus performs tomography by the following procedure. First of all, an X-ray tube and multi-channel X-ray detector are arranged to oppose each other through an object to be examined. The object is then irradiated with an X-ray beam from the X-ray tube while the X-ray and X-ray detector are rotated around the object through 360°. The X-rays transmitted through the object are detected by the X-ray detector. At this time, the intensity of X-rays emitted from the X-ray tube is constant (i.e., the tube voltage and tube current in the X-ray tube are constant). Note that the X-rays emitted from the focal point of the X-ray tube is collimated into a fan-shaped X-ray beam. The spread width of an X-ray beam is determined in accordance with a slice thickness and the like.
A set of projection data detected by the X-ray detector at a given rotational angle centered on the body axis of an object to be examined is called a view. Measurement of the amounts of X-rays transmitted in a plurality of view directions which is done while the X-ray and X-ray detector are rotated around the body axis of the object is called scan. By performing reconstruction processing for the projection data of a plurality of views obtained by this scan using a high-speed arithmetic unit or the like, a tomographic image of the object can be obtained.
A cross-section of the human body taken from a chest portion to a waist portion can be regarded as a nearly elliptic shape, as shown in FIG. 1. When, therefore, the human body is imaged by an X-ray CT apparatus, the detected amount of X-ray transmitted varies depending on the emission angle. This is because the amount of X-rays absorbed by an object P to be examined when the object is irradiated with X-rays from an angular position θ1 (the major-axis direction of the ellipse) differs from that absorbed by the object when the object is irradiated with X-rays from an angular position θ2 (the minor-axis direction of the ellipse). In particular, the detected amount of X-rays transmitted on the plane side of the human pelvis portion or the like considerably differs from that on the side surface side.
FIG. 2 is a graph showing the relationship between the angular position of the X-ray tube and the amount of X-rays transmitted in a case wherein the human body is imaged by a conventional X-ray CT apparatus. The abscissa represents the emission angle of X-rays with respect to the object P (i.e., the angular position of the X-ray tube); and the ordinate, the amount of X-rays transmitted. As is obvious from FIG. 2, the amount of X-rays transmitted in the major-axis direction θ1 of the ellipse is smaller than that in the minor-axis direction θ2 of the ellipse. In addition, the amount of X-rays transmitted periodically changes along the entire circumference of the object P regarded as an elliptic shape.
Variations in the amount of X-rays transmitted with respect to this X-ray emission angle cause variations in S/N ratio with respect to the emission angle. That is, the S/N ratio is high at a portion where the amount of X-rays transmitted is large, whereas the S/N ratio is low at a portion where the amount of X-rays transmitted is small. Consequently, when a tomographic image is to be generated from views obtained by this scan, the overall S/N ratio of the tomographic image becomes low.
In order to solve this problem, for example, the following two techniques are conceivable.
One is a technique of increasing the overall S/N ratio of an image by increasing the amount of X-rays transmitted as a whole. In order to realize this, the intensity of X-rays to be emitted must be increased. In this case, however, an excessive amount of X-rays is applied to a portion originally having a high S/N ratio. This therefore causes an increase in radiation dose.
The other is a technique of increasing the overall S/N ratio of an image by controlling the tube voltage or tube current in the X-ray tube in accordance with the rotational angle (X-ray emission angle) of the X-ray tube. With this technique, the object P can be scanned with a constant amount of X-rays transmitted on one slice. This technique is effective in obtaining a tomographic image of one slice. If, however, tomography is continuously executed many times by helical scan as in recent years, the radiation dose cannot be satisfactorily reduced. This is because, as shown in FIG. 3, the object P has uneven thickness in the body axis direction, so that the amount of X-rays transmitted at a given portion is large while that at another portion is small depending on the position of the object P in the body axis direction.
Recently, further improvements have been made on the latter technique. For example, a technique of making the amounts of X-rays transmitted in the respective angular directions around an object to be examined and at the respective positions in the body axis direction almost uniform in an X-ray CT apparatus designed to perform helical scan has been proposed by Tanaka (Japanese Patent No. 2768932). According to this technique, scanography is executed in two directions with different rotational angles (e.g., plane direction and side surface direction) before tomography by helical scan, and a proper X-ray emission amount pattern is estimated from this scanography. In executing helical scan or the like, the tube current in the X-ray tube is controlled to make the amount of X-rays emitted from the X-ray tube coincide with the estimated pattern in accordance with the rotational angle of the X-ray tube with respect to the object and the position of the X-ray tube in the body axis direction. Note that scanography is an imaging method in which an X-ray tube is translated relative to an object to be examined in the body axis direction while X-rays are emitted in a state wherein the X-ray tube is fixed at a predetermined angular position with respect to the object, and the amount of X-rays transmitted is detected by an X-ray detector.
In the above improved technique, however, in order to obtain a proper X-ray emission amount pattern in helical scan, scanography must be executed twice. This therefore increases the radiation dose to the object. In addition, since scanography must be executed twice, the operation efficiency deteriorates.
It is, therefore, an object of the present invention to provided an X-ray CT apparatus which reduces the radiation dose to an object to be examined and improves operation efficiency and image quality by determining proper imaging conditions and executing scan in accordance with the determined conditions.